In the field of manufacture of electronic devices, a plasma processing apparatus is used for processes such as etching, deposition and the like for a target object to be processed. The plasma processing apparatus includes a process container and a mounting table, for mounting thereon a target objects to be processed which is installed in the process container. The mounting table generally includes an electrostatic chuck to which a voltage is applied. The target object is attracted to and held on the electrostatic chuck by a coulomb force generated by the voltage.
In the plasma processing apparatus, the surface condition of the electrostatic chuck varies over time. For example, reaction products generated during plasma processing may be deposited, as an alterated layer, on the surface of the electrostatic chuck. When the alterated layer is formed on the surface of the electrostatic chuck, the surface of the electrostatic chuck may not be neutralized even after the application of the voltage is stopped. Under a state where the surface of the electrostatic chuck is electrified, if a target object is detached from the electrostatic chuck, a load may exert on the target object, which may result in misalignment or crack of the target object. Therefore, a process of neutralizing the electrostatic chuck before the target object is detached has been conventionally performed.
In order to neutralize the electrostatic chuck, a voltage having the opposite polarity to a voltage for attraction is typically applied to the electrostatic chuck. This technique is disclosed in Japanese Patent Application Publication No. 2004-40046.
However, in the technique disclosed in Japanese Patent Application Publication No. 2004-40046, it is difficult to neutralize the electrostatic chuck since it cannot correctly detect an amount of charge of the electrostatic chuck. If the electrostatic chuck cannot be neutralized, the target object may be misaligned or cracked when the target object is detached from the electrostatic chuck.
Some electrostatic chucks may have an upper surface having a base portion and a plurality of convex portions projecting from the base portion. Such an electrostatic chuck, the target object is mounted on apexes of the plurality of convex portions such that the back surface of the target object is in contact with the apexes.
The present inventors have discovered that, under the state where the target object is attracted and held on the electrostatic chuck, the back surface of the target object is elastically deformed and a distance between the back surface of the target object and the base portion of the electrostatic chuck is varied depending on an attraction force of the electrostatic chuck. Based on this discovery, the present inventors have found that the attraction force (i.e., an amount of charge) of the electrostatic chuck can be indirectly detected by calculating the distance between the back surface of the target object and the base portion of the electrostatic chuck.